src: remove unused function declaration

PR-URL: https://github.com/nodejs/node/pull/8878


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>